The present invention relates to centrifugal countergravity casting of metals and alloys.
A countergravity casting process for making investment castings in gas permeable ceramic shell molds is described in U.S. Pat. Nos. 3,863,706; 3,900,064; 4,589,466; and 4,791,977. The ceramic shell mold is formed by the well known xe2x80x9clost waxxe2x80x9d process and includes an upstanding riser passage around which are located arrays of mold cavities in the shape of the cast articles to be made. The mold cavities are located along the length of the riser passage from proximate a bottom to a top thereof, and each mold cavity communicates to the riser passage via one or more relatively narrow feed gate passages depending upon the configuration of the mold cavity. The ceramic mold is disposed in a vacuum container, and a fill tube is communicated to the bottom of the riser passage and extends out of the container for immersion in an underlying pool of molten metal. A relative vacuum (subambient pressure) is established in the container when the fill tube is immersed so as to draw molten metal upwardly into the riser sprue and into the gate passages and mold cavities. In typical commercial production practice, the molten metal in the gate passages and mold cavities typically is solidified before the vacuum in the container is released, although U.S. Pat. No. 3,863,706 discloses releasing the vacuum in the container after the molten metal in the gate passages and mold cavities has solidified to produce individual cast articles and to allow return of still molten metal in the riser passage to the underlying pool for reuse.
The ceramic shell mold can be disposed in a particulate support media, such as dry foundry sand, in the vacuum container as described in U.S. Pat. No. 5,069,271. The thickness of the shell mold wall can be reduced by use of the support media in the vacuum container. The container is evacuated using a vacuum head that also compresses the support media about the shell mold as a subambient pressure is established in the container.
Countergravity casting methods result in a large variation in the time that it takes to fill identical mold cavities located at different elevations along the length of the upstanding riser sprue. Depending on such.parameters as location of the mold cavity along the riser passage, gas permeability of the particulate support media, gas permeability of the ceramic shell mold, rate of evacuation of the container, final vacuum level in the container, and others, the time needed to fill mold cavities of the same shell mold can vary by a factor of two or more. For example, the lowermost mold cavities take the longest to fill with molten metal and the uppermost mold cavities take the shortest time. Delayed filling of the lowermost mold cavities can result in incomplete filling thereof with molten metal. Rapid filling of the uppermost mold cavities can result in entrapped gas defects in the solidified cast articles formed in those mold cavities. Unfortunately, attempts to ameliorate one of the these problems (delayed filling or rapid filling) further promotes the detrimental effects of the other.
Countergravity casting methods also result in a large variation in the pressure in the mold cavities. The pressure in each mold cavity is equal to atmospheric pressure pushing on the surface of the molten metal pool when the container is evacuated minus the static pressure of the molten metal in the riser passage that acts counter to the atmospheric pressure on the pool surface. Thus, the pressure in the mold cavities depends on their elevation along the length of the riser passage; more particularly, the pressure depends on the difference in elevation between the surface of molten metal pool and the gate of the mold cavity. The taller the shell mold, the greater is the pressure variation among mold cavities along the length of the sprue. The pressure reduction increases shrinkage and entrapped gas defects in mold cavities located higher up along the riser.
When the molten metal drawn upwardly reaches the closed, upper end of the riser passage, the upper mold cavities may not yet be completely filled with molten metal. When the riser passage is filled to the top end, the molten metal impacts the top end of the riser passage such that there thus is a resulting surge in pressure differential across the gate passages of the upper mold cavities that causes the upper mold cavities to fill too quickly. Much of any gas entrained in the molten metal in the riser passage is carried into the mold cavities where it can remain in the solidified cast articles formed in the mold cavities.
To prevent flow-back of molten metal from the mold cavities and gate passages, the fill tube is kept immersed in the molten pool sufficiently long for the molten metal to solidify in the mold cavities and gate passages. Having to maintain immersion of the fill tube slows the casting cycle time and requires that the mold follow the dropping level of molten metal in the pool such that the mold become more and more exposed to the induction field that is used to heat the pool. The induction field can retard, or reverse, solidification in the mold and distort the container proximate the fill tube in a manner that permits airflow into the lower mold cavities. Gating design becomes a struggle between having gate passages with sufficient volume to feed the mold cavities, yet narrow enough to solidify molten metal in a timely manner therein. Moreover, these constraints on gate design limit the size of cast articles that can be made by the process described in U.S. Pat. No. 3,863,706 to usually less than one pound.
In countergravity casting of large articles, modifications have been made to the method and apparatus to capture molten metal in the riser passage. For example, one modification disclosed in U.S. Pat. No. 4,589,466 involves pinching shut the metal fill tube through which the molten metal is drawn into the mold after the mold is filled. A ceramic coated ball valve or stopper in the fill tube also have been used to this end. Such process is described in U.S. Pat. No. 3,774,668. U.S. Pat. No. 4,961,455 discloses a refinement of the xe2x80x9ccheck valvexe2x80x9d by proposing the use of a ferromagnetic, ceramic coated ball forced by magnets to seal the tube through which the melt is drawn. Use of a siphon-trap in the fill tube and inverting of the mold after casting also have been attempted to this end. Use of a ceramic strainer as described in U.S. Pat. No. 4,982,777, or a strainer and convoluted passageway combined as described in U.S. Pat. No. 5,146,973, or a siphon-like passageway alone in the fill tuber as described in U.S. Pat. No. 5,903,762 to retard alloy flow-back from the riser while the mold is inverted have been disclosed. These modifications partially obstruct flow into the riser and result in slow mold filling. All of these processes require solidification of the molten metal in the riser passage, resulting in relatively low utilization of molten metal. In all of these processes, the geometry of the casting, that is, the number of patterns that can be arranged around the riser, is limited by the necessity of leaving sufficient space around the riser to facilitate the separation of the castings from the riser. U.S. Pat. No. 4,112,997 proposes the inclusion of xe2x80x9cstabilizingxe2x80x9d screens in the gates. It is claimed that the screens will retain alloy in the mold cavities after pressure in the mold chamber is returned to ambient. If indeed practical and economical, this process would remove the geometric constraint imposed by the cutting of the castings from the solidified riser, by eliminating the riser itself.
An object of the present invention is to provide a centrifugal countergravity casting method and apparatus that overcomes the above described problems and compromises associated with filling of mold cavities at different elevations along the length of the riser passage.
Another object of the invention is to provide a casting method and apparatus for trapping molten metal or alloy in the mold cavities and gates through centrifugal action, while allowing for the voiding of the molten metal from the riser, resulting in castings unattached to the riser.
The present invention provides in one embodiment method and apparatus for countergravity casting a plurality of articles wherein a ceramic mold is provided having an upstanding riser passage and a plurality of mold cavities disposed along a length of the riser passage at different elevations, each mold cavity communicating to the riser passage via a gate passage, wherein molten metal is caused to flow upwardly from a source into the riser passage for supply to the mold cavities via their gate passages, wherein the mold is rotated so that molten metal that resides in the gate passages is subjected to centrifugal force in a direction toward the mold cavities, and wherein molten metal in the riser passage is drained to empty the riser passage before molten metal in the mold cavities and the gate passages completely solidifies, leaving the gate passages at least partially filled with molten metal for supply to the mold cavities in response to shrinkage as molten metal therein solidifies while the container is rotated. The molten metal in the mold cavities is solidified while rotating the container to form a plurality of individual solidified cast articles in the mold cavities. Rotation of the mold can be terminated after molten metal solidifies in the mold cavities. Much higher yields of metal or alloy of 80% and above are achievable by practice of the invention. A much greater number and larger size of articles with increased density due to reduced shrinkage can be cast in practice of the invention.
When the riser passage is drained, ambient pressure is present therein such that still molten metal partially filling the gate passages and filling the mold cavities is subjected to ambient pressure plus pressure due to centrifugal motion of the container in a manner that increases density of cast articles by reducing shrinkage. The molten metal residing in the gate passages solidifies faster once the riser passage is drained to reduce or prevent flow back of molten metal from the gate passages.
In a preferred embodiment of the invention, the steps of causing the molten metal to flow upwardly into the riser passage and of rotating the mold are conducted concurrently during filling of the mold cavities when casting molten metals that are prone to shrinkage problems. These steps optionally can be conducted sequentially with mold rotation being initiated after the molten metal is caused to flow upwardly to fill the mold cavities. The mold can be rotated about a longitudinal axis of the mold or an axis offset from and substantially parallel to a longitudinal axis of the mold.
In another embodiment of the invention, each mold cavity is elongated in the direction of the riser passage and is positioned (e.g. tilted) relative to the riser passage such that a theoretical melt surface provided by mold rotation passes only through the gate passages during draining of the riser passage but does not pass through the mold cavities so that molten metal is not voided from the mold cavities as the riser passage is drained.
In another embodiment of the invention, each mold cavity is elongated in the direction of the riser passage and is connected thereto by a plurality of gate passages at different elevations on the riser passage. Molten metal is initially solidified at regions in the mold cavity between the gate passages so to confine still molten metal in a plurality of more or less discrete compartments in the mold cavity between the solidified regions such that the gate passages partially filled with molten metal will supply still molten metal therein to a respective compartment in response to shrinkage as molten metal solidifies while the container is rotated.
The invention can be practiced using gas permeable molds and gas impermeable molds. The invention is further beneficial in casting gas impermeable molds to reduce or eliminate entrapped gas in the mold cavities thereof.
In a particular apparatus embodiment of the invention, the ceramic mold is supported in a particulate medium, such as for example dry foundry sand, in an evacuable container. The container is evacuated to subambient pressure to force molten metal upwardly into the mold riser passage and rotated by a rotary drive mechanism disposed on a support frame on which the container is mounted for rotation.
The present invention envisions in still another embodiment of the invention replacing the ceramic mold with a fugitive pattern in the container. The fugitive pattern is supported in a particulate medium in the container and includes an upstanding riser passage-forming portion and a plurality of mold cavity-forming portions disposed along a length of the riser passage-forming portion at different elevations. Each mold cavity-forming portion communicates to the riser passage-forming portion via a gate passage-forming portion. The molten metal progressively destroys the pattern to form a riser passage, mold cavities and gate passages in the particulate medium.
The invention achieves more uniform time of filling of the mold cavities at all elevations as well as more uniform pressure in the mold cavities and reduction of pressure surge proximate the upper mold cavities, reducing gas entrapment in the cast articles.
Advantages and objects of the present invention will be better understood from the following detailed description of the invention taken with the following drawings.